1. Field of Invention
The present invention relates to a soft piezoelectric ceramic composition and, more particularly, to a soft piezoelectric ceramic composition sinterable at a reduced temperature with high piezoelectric parameters and a piezoelectric device using the same.
2. Description of the Prior Art
With a successful application of piezoelectric actuators for valve control of fuel injection system in diesel engine, multi-layer piezoelectric actuators finally find a major application. Soft PZT (lead zirconium titanate, Pb(Zr Ti)O3) materials with a big piezoelectric constant, d33, are generally used for piezoelectric actuators to get a larger displacement. Monolithic multi-layered structures are always chosen to get a sufficient displacement at a low input voltage as in automobiles.
When producing multi-layer piezoelectric actuators with conventional soft PZT materials, expensive Agxe2x80x94Pd or Pt alloys are usually used for internal electrodes due to a high cofiring temperature. Pd contents are dependent on the cofiring temperature and usually in the range of 20xcx9c70% by weight when sintering at 1150xcx9c1300xc2x0 C. Therefore, there is a big technical push to decrease sintering temperature of piezoelectric ceramics, preferably below the melting point of Ag (Silver), to reduce a cost of internal electrodes in multi-layer piezoelectric actuators.
Soft piezoelectric ceramic materials have typical characteristics of:
high piezoelectric constant
high electromechanical coupling coefficient, Kp
high dielectric constant
high dielectric loss
low mechanical quality factor, and
high residual polarization with a low coercive field.
In PZT based materials, these properties are achieved by substituting (Zr4+ or Ti4+) with ions of a higher valence such as Nb5+, Ta5+, W6+, etc. or by substituting Pb2+ with La3+. However, sintering temperature of conventional soft piezoelectric ceramic materials are in the range of 1150xcx9c1200xc2x0 C. (Refer to the following: 1) H. Zheng et. al., xe2x80x9cEffects of Octahedral Tilting on the Piezoelectric Properties of Strontium/Barium/Niobium-doped Soft Lead Zirconate Titanate Ceramicsxe2x80x9d, J. of Am. Ceram Soc. 85[9], 2337(2002). 2) U.S. Pat. No. 5,423,995) Therefore, expensive Agxe2x80x94Pd alloys have to be used to build multi-layered piezoelectric actuators by cofiring green sheets with internal electrodes.
Sintering temperature of PZT materials can be lowered by adding sintering aids such as B2O3, V2O5, Bi2O3, CuO, which form liquid phase enhancing densification at a reduced temperature. (Refer to the followings; 1)U.S. Pat. No. 5,423,995, 2)U.S. Pat. No. 5,433,917, 3)U.S. Pat. No. 5,792,379, 4) K. Murakami et. al. xe2x80x9cMorphotropic Phase Boundary and Microstructure of Low-Temperature sintered PZT ceramics with BiFeO3 and Ba(Cu0.5W0.5)O3xe2x80x9d. Proceedings of the Ninth IEEE International Symposium on Applications of Ferroelectrics 5) X. Wang et. al., xe2x80x9cThe Mechanism of Low Temperature Sintering PZT Ceramics with additives of Li2O3xe2x80x94Bi2O3xe2x80x94CdOxe2x80x9d, Proceedings of Eighth IEEE International Symposium on Applications of Ferroelectrics.) However, these sintering aids often remain as glassy phases in the sintered ceramics, increasing dielectric loss tangent and deteriorating piezoelectric parameters. In addition, these liquid phases formed during sintering react easily with Ag or Ag/Pd electrode and make it difficult to cofire multi-layered piezoceramic elements.
The present invention is intended to solve the above-described problems of the conventional techniques.
Therefore, an object of the present invention is to provide soft piezoelectric ceramic composition which can be sintered at a reduced temperature, preferably further below the melting point of pure Ag so that they can be co-fired with pure Ag electrode.
Another object of the present invention is to produce piezoelectric device using the same materials.
Still another object of the present invention is to produce multi-layered piezoelectric device using the same materials.
In achieving the above objects, the soft piezoelectric ceramic composition according to the present invention includes: [(Pb1-m-nSrmBan)(1-y)Biy][(ZrxTi1-x)1-a-bNiaWb]O3+pCdO, wherein m, n, x, y, a and b are molar ratio, p is wt % and are in ranges, respectively,
0.00% mxe2x89xa60.18, 0.00xe2x89xa6nxe2x89xa60.18,0.00xe2x89xa6(m+n)xe2x89xa60.21,0.40xe2x89xa6xxe2x89xa60.60 0.00xe2x89xa6yxe2x89xa60.04, 0.00xe2x89xa6axe2x89xa60.02, 0.00 less than b less than 0.02, 0.00 less than pxe2x89xa62.00. 
In another aspect of the present invention, the piezoelectric ceramic device according to the present invention comprises: a piezoelectric ceramic comprising said piezoelectric ceramic composition, sintered below 980xc2x0 C.
In still another aspect of the present invention, the multi-layered piezoelectric ceramic device according to the present invention comprises: a piezoelectric ceramic layer, wherein the piezoelectric ceramic layer comprising said piezoelectric ceramic composition, co-fired with pure Ag internal electrode below 960xc2x0 C.
In still another aspect of the present invention, the multi-layered piezoelectric ceramic device according to the present invention comprises: a piezoelectric ceramic layer, wherein the piezoelectric ceramic layer comprising said piezoelectric ceramic composition, co-fired with Agxe2x80x94Pd internal electrode below 1050xc2x0 C.
According to this invention, it is possible to provide soft piezoelectric ceramic composition which have high piezoelectric constants and electromechanical coupling coefficients and which can be sintered at a reduced temperature below 1000xc2x0 C. and further soft piezoelectric ceramic materials which can be co-fired with pure Ag electrode below 960xc2x0 C.
Based on the piezoelectric ceramic compositions according to this invention, multi-layer piezoelectric actuators are built with pure Ag internal electrode, saving a cost of electrode as much as 90% compared to Agxe2x80x94Pd (30%) internal electrode.